That is correct. And nobody is allowed to hit/block a player whose helmet came off. If the ball carrier's helmet comes off the ball is dead at that spot.

Seen this happen in a Mooney/Harding game a few years ago. Braylon Heard lost his helmet on his way to a long touchdown run. Play was called dead at the spot where his helmet came off. Crowd wasn't happy, but its for the safety of the ball carrier.